Extracellular triacylglycerol lipases are important enzymes of lipid digestion and lipoprotein metabolism. Two of these, pancreatic triacylglycerol lipase and lipoprotein lipase, are unique in that their activities are controlled by low molecular weight protein cofactors, colipase and apolipoprotein CII, respectively. These cofactors are known to absorb to the lipid-water interfaces at which these lipases function, e.g. the surface of a lipoprotein, in competition with other surface-active molecules, e.g. bile salts, albumin and other apolipoproteins. Each cofactor also engages in protein-protein interactions with its respective lipase. In this way they enable the absorption of the lipase to the surface and, hence, allow it to catalyze the hydrolysis of lipids residing in that interface. I have discovered that the interaction of these cofactor proteins is not driven by general surfactancy but exhibits specificity. This specificity favors their adsorption to interfaces containing the substrates of lipolysis in preference to matrix lipids, like phosphatidylcholine, and implies that these substrates will be concentrated in the vicinity of the cofactor in the plane of the interface. The aims of this project are to test the hypotheses that this specificity has a steric origin and that the lateral concentration of substrate by cofactor protein regulates both lipase adsorption to the interface and the pool of substrate accessible to adsorbed lipase. The steric hindrance hypothesis will be tested by measuring the strength of lipid-cofactor interactions as a function of lipid structure in adsorbed and spread monomolecular films containing substrates and matrix lipid. For selected films the affinity and extent of lipase adsorption and the availability of substrate to adsorbed lipase will also be measured. Correlation of the lipase adsorption and catalysis results with the lipid-cofactor interaction data will provide a test of the second hypothesis. These results will increase our understanding of how these and other cofactor proteins, e.g. apolipoprotein AI, regulate interfacial, enzyme-catalyzed reactions and will increase our fundamental understanding of how lipid-protein interactions regulate lipid lateral organization in interfaces.